Galileo is a state-of-the-art Global Satellite Navigation System (GNSS) which is currently being developed by the European Union (EU) and the European Space Agency (ESA). The system is designed to provide a highly accurate and guaranteed global positioning service under civilian control. It will be also interoperable with other GNSSs, such as the US Global Positioning System (GPS) and the Russian GLONASS system.
The Galileo mission and services have been elaborated during the initial definition phase in consultation with user communities and the EU Member States. Galileo will provide four basic “satellite-only” services to worldwide users. These include the Open Service (OS), the Commercial Service (CS), the Public Regulated Service (PRS), and the Search and Rescue Service (SAR). Additional details about the Galileo services can be found in [1].
The successful provision of these services depends on the system capability of broadcasting up-to-date navigation messages to the users. In particular, the different Galileo services are supported by four types of navigation messages: the Freely accessible (F/NAV), the Integrity (I/NAV), the Commercial (C/NAV), and the Governmental (G/NAV) navigation messages. The structure of these messages, together with the supported services, is described in many textbooks (e.g. [2]). As for other GNSSs, the content of each message is first prepared at the Ground Segment (GS) and then uplinked to the Space Segment (SS). Once the uplink is complete, the Galileo satellites (“Space Vehicles,” SVs) broadcast the message on a global scale.
A key element of this process is the “dissemination function”, i.e. the GS function which performs the uplink of the messages to the SS. This function uses a dedicated C-band channel and a network of ground antennas, available at different Uplink Station (ULS) sites. Furthermore, this uplink function is performed according to a given set of “dissemination requirements,” which are determined based on the service level performance associated with each of the four Galileo services, [3]. For example, a necessary condition for the OS nominal accuracy is that the relevant messages are regularly updated. This leads, for each SV, to a requirement for the maximum interval (e.g. 100 minutes) between two successive contacts between the SV and one of the ULS antennas available at the GS.
More generally, three types of dissemination requirements are currently being considered for the provision of Galileo services. In particular, these consist of:                4. A Navigation Data Refresh Rate (NDRR) requirement, which specifies the maximum interval between two successive contacts between each SV and the GS;        5. A Minimum Contact Duration (MCD) requirement, which indicates the minimum duration of each contact; and        6. One or more Link Availability (LA) requirements (also referred to herein as LA specifications), which denote the percentage of time when a generic user is in view of one (or more) “connected” SVs, at a minimum elevation angle with respect to the user. In this context, “connected” indicates that there is a contact between the SV and one of the ULS antennas.        
The NDRR is provided to ensure that the SVs are maintained with current (up-to-date) data. The MCD requirement is provided to ensure that the contact between the SV and the ULS is long enough to transmit the desired amount of data. The LA requirements are generally used for providing more sophisticated services, such as CS and SAR, which involve real-time communications, via the ULS antenna and the SV, between some terrestrial system and a user. A typical example is the SAR Return Link Alert Service, which provides a user in distress with an acknowledgment message informing them that their alert has been detected and located. Note that the NDRR and MCD requirements have broad relevance for GNSS systems in general, whereas the LA requirements are less common, since existing GNSSs may have only limited support (if any) for more specialized services.
In order to meet the above dissemination requirements, the dissemination function is performed according to a specific schedule which is computed by a dedicated Uplink Scheduling (ULSC) algorithm. In the specific case of the Galileo system, this is implemented at the Mission and Uplink Control Facility (MUCF) of the Galileo GS. In addition to the dissemination requirements, the ULSC algorithm takes into account the predicted orbits of the SVs and the actual availability of both the SVs and the ULS antennas (for example, if a particular ULS antenna is scheduled to be unavailable at a particular time because of planned maintenance). Based on this information, the ULSC algorithm computes a contact plan (or schedule) for ten sidereal days, which is the Ground Track (GT) repetition period of the Galileo constellation. The plan unambiguously indicates, for each “time sample” (or “epoch”) and for each ULS antenna, the status of the antenna, i.e. whether the antenna shall be connected or not to a given SV. The existing ULSC algorithm currently available at the MUCF was primarily developed to address the NDRR and the MCD requirements, together with the original LA requirements to support the former Safety of Life (SoL) service. However, further development of services for Galileo, such as CS, has led to the need to support more complex LA requirements.